In a Fourdrinier or paper making machine, it is usual to flow a dilute aqueous slurry of fibrous material onto a continuously moving continuous wire. Ordinarily, the fibrous material is cellulose or wood pulp and the slurry may, if desired, contain various paper additives such as sizing or materials to improve strength or other properties. After the slurry is laid down on the wire, the water is drained off, the resulting web of paper is dried and, if desired, further materials or coatings may be placed on the paper.
One of the problems associated with the paper making process is the removal of the relative large quantity of water from the paper fibers. The greatest part of this water is removed by drainage through the Fourdrinier wire. It is usually, thereafter, further removed by passing the wire over a suction box which draws the water through the fiber and wire. Then it is the usual practice to press the paper, similarly to remove additional water, after which the remainder of the water is removed by evaporation. In the earlier stages, the paper fibers which have been freshly laid down are obviously and visibly wet. As this newly formed paper web advances through the machine, it reaches a point known as the "dry line" at which the paper looks dry. At this point, the paper will usually contain several times as much water as fibrous material and it is the removal of this residual quantity of water which is most troublesome.
If a significant improvement can be made in the removal of this water, one or both of two benefits can be achieved. In the one instance, operating with existing equipment whose speed is not limited by other factors, it is possible to increase the speed of the wire and thus increase the speed of the entire paper making process. Ordinarily, a paper machine operates at a speed ranging between extremes of about 50 to about 5,000 feet per minute, but more usually from about 600 feet a minute with a bulky paper to roughly 3,000 feet per minute, depending of the nature of the paper, its thickness or basis weight, the added materials and a number of other factors, but a typical speed is about 1,500 feet per minute. If a significant increase in speed is achieved, the result is that a greater quantity of paper is made on the machine in a given period of time and with a given amount of labor producing a finite cost reduction in an extremely cost conscious industry. Accordingly, the reduction in cost of the finished paper product can be quite significant as a consequence of even a very small increase in the rate of removal of its water.
In other cases, it is possible to achieve very substantial reductions of cost in other forms. For example, if new equipment is being constructed, it can be made considerably shorter if a lesser quantity of water must be removed after the paper has passed through the preliminary steps of water removal and very important capital investment can be saved, again in a highly cost conscious industry.
One of the points of opportunity for increased efficiency or effectiveness of removal of water from newly formed paper is at a point approximately at the dry line, or the point at which it is usual to pass the machine wire over one or more suction boxes to remove that portion of the water which can be drawn from the paper web through the wire and into a drainage mechanism. Water or moisture that remains in the paper beyond this point is usually removed by pressing and by evaporation requiring very significant amounts of heat.
It has previously been known that water can be removed either more completely or more quickly from the paper web at the suction boxes if the temperature of the paper and the water is significantly elevated. The reason for this improvement has not been fully understood but it is believed that it is a consequence of either or both of two factors. If there is substantial increase in the temperature of the water, there is also a very substantial decrease in its viscosity with the result that it flows more quickly away from the paper fibers. For example, raising the temperature of water from about 110.degree.F to about 140.degree.F just about halves its viscosity. It is also true that with a significant increase in temperature, there is a measurable decrease in the surface tension of the water, with the expected consequence that more of the water can be removed at the suction boxes. Whatever the reason, hotter water is removed both more quickly and more nearly completely. The appropriate increase in the temperature has, in the past, been accomplished by heating the moist paper with steam. For this purpose, it is most desirable that the steam be essentially saturated so that the largest quantity of heat can be transferred from the vapor to the paper web by condensation. The use of steam in paper manufacture is a common procedure, and is commonly not well understood. For example, Dupasquier U.S. Pat. No. 2,642,314 uses a steam shower for control of the surface characteristics. A few years later the same Dupasquier, in a later U.S. Pat. No. 2,809,867, used steam in an upward stream from beneath the wire, and found this advantageous. Goyette, U.S. Pat. No. 2,949,239 also uses steam in a paper making machine for an unstated purpose, directed in a converging flow between rolls. Furthermore, steam has been used, and is now used in practice to assist in drying the paper web. The method and apparatus most generally now in use to heat the paper web and the water contained in the web employs a relatively large and heavy steam box placed directly above the paper web at the suction box location. This steam box has a number of slits in its bottom and a number of V-shaped troughs, both running across the direction of motion of the paper. Steam is fed to this box where, hopefully, any condensed moisture falls into the troughs and can be drained off while steam is forced through the slits and down toward the paper. Such a system is illustrated in Dupasquier U.S. Pat. No. 2,838,982.
This commonly used system has a number of drawbacks, some of which are a consequence of mechanical ackwardness and some of which are process related and are likely to bring about machine stoppage or to produce water spotting on the paper from condensed moisture which is not fully separated from the steam.
For example, the slits of the commonly used equipment, which are quite narrow and which are positioned quite close to the paper, can and do become clogged or partially clogged so as to interfere with the flow of steam onto the paper. In addition, the troughs tend to become partly or largely filled with water which is entrapped in the steam and may be spattered onto the paper, producing permanent water spots on the paper web. Some of these difficulties occur primarily when a machine is first started up and, accordingly, the problems are partly alleviated by raising the relatively heavy steam box a number of inches above the paper until the machine is operating at its equilibrium condition. Attempts to remedy these flaws in the prior systems have been made and have failed generally because efforts to make the steam box smaller and lighter have usually resulted in uneven end-to-end performance of the equipment. In addition, the results, while being a substantial improvement over results achieved without such heating of the web, have still produced less removal of water than is desired, and a great deal of expense is involved in subsequent heat-drying of the web. The horns of the dilemma have appeared to be the incompatible characteristics that reducing the flow of steam reduces the amount of heat which can be transferred to the paper and thus limits the amount of water that can be drawn from the paper at the suction boxes, while on the other hand increasing the flow of steam seems inevitably to lead to water spotting of the paper.